A transmission device is an apparatus that changes torque and the number of rotations by interposing a single or a plurality of gear wheels and the like in between an input means (input axis) and an output means (output axis) such as an axle and by adjusting and controlling the number of rotations and the like of force having been input. Transmission devices are used in, for example, automobiles and the like, and a number of non-step transmission devices capable of continuously changing in a non-step manner and transmitting transmission gear ratios from input means have also been developed.
As one of such the transmission devices, a transmission device that controls rotations of an output means by using an internal/external gear wheel gear-coupled to an input means through internal cogs and to the output means through external cogs and by making the internal/external gear wheel revolve around a circumference of the input means has been developed by the inventor of the present invention (see Patent Literature 1). According to this transmission device, speed of rotations of the output means can be changed by just adjusting revolving speed of the internal/external gear wheel, which enables a reduction in the number of components forming the transmission device, thereby enabling achievement of a simplified and compact structure of the transmission device.